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Boko Haram's insurgency has become more brutal in the past year

Nigeria's militant Islamists are in control of the key town of Damboa in north-eastern Nigeria, a local vigilante leader has told the BBC.

The vigilante force defending the town fled on Sunday, and Islamist group Boko Haram's black flag is now flying over Damboa, he said.

At least 40 people were killed when Boko Haram attacked Damboa on Friday, the vigilante leader added.

The group has been fighting since 2009 to create an Islamic state in Nigeria.

In April, it sparked international outrage by abducting more than 200 girls from their boarding school in Chibok, in Borno state, like Damboa.

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The military is said to lack the equipment to fight Boko Haram

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A global campaign has been waged to secure the release of the girls

The BBC's Chris Ewokor in the capital, Abuja, says when Boko Haram seized towns and villages in the past, it was driven out by the military.

However, government forces have failed to launch an offensive to recapture Damboa, he says.

'Checkpoints'

It is one of the biggest towns in Borno state and a busy trading centre for people from neighbouring villages.

Military spokesman Chris Olukolade said government forces were "firming up" their deployment to the area.

The military would not "concede any portion of this country" to Boko Haram he said.

Meanwhile, a military helicopter flying in Borno state's Bama area crashed after developing a technical fault, Mr Olukolade said.

He did not give any details of casualties.

During the fighting in Damboa, some electric installations were damaged and this has left the regional capital, Maiduguri, without electricity for the past three weeks, a local resident has told the BBC.

Damboa is about 85km (53 miles) from Maiduguri, the headquarters of Boko Haram before it was driven out by government forces last year.

The vigilante leader, who asked not to be named for fear of reprisals, told the BBC that Boko Haram gunmen had set up checkpoints on roads to Damboa and were charging motorists a toll fee.

Roads from Damboa lead to Boko Haram's base in the Sambisa forest, where the abducted schoolgirls are believed to be held, as well as Chibok.

The vigilante forces abandoned the town after running out of ammunition.

Boko Haram's flag had been hoisted outside the home of Damboa's traditional ruler, and the town's entrance, he said.

The group had also seized the military barracks, which government soldiers had abandoned after an earlier attack by the militants, the vigilante leader added.

The government has been under heavy criticism for its failure to end the insurgency or to secure the release of the Chibok girls.

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The girls were seized from their hostel late at night

President Goodluck Jonathan declared a state of emergency last year in Borno, Yobe and Adamawa states, saying he would give the military extra powers to fight Boko Haram.

However, the insurgency has become more deadly since then.

New York-based Human Rights Watch says at least 2,053 civilians have been killed in an estimated 95 attacks during the first half of 2014, compared with 3,600 deaths in the first four years of the conflict.